Kenwood KT-880D

The Kenwood KT-880D is an inexpensive AM/FM tuner with remarkable performance. It is sensitive. It is one of the few tuners immune to HD Radio self-noise without modification. Distortion is extremely low, and unlike many tuners, doesn't rise for weak signals. Finally, resistance to front-end overload rivals that of tuners with balanced mixers. The KT-880D uses simple, straightforward circuitry that works so well you wonder why designers ever bother with fancier circuits.

I tested two KT-880Ds. The first surprise was how low their distortion was out of the box. Before adjusting anything, neither tuner had any harmonics above −70 dB. After alignment, 1-kHz stereo THD for both was just 0.02%. When I reduced the RF level, the noise floor simply rose and covered the distortion products. THD for some tuners can exceed 1% for weak signals. Distortion did increase at higher signal levels, but not alarmingly. I measured 0.06% THD at 102 dBf, a much smaller increase than some tuners show.

Kenwood did something clever with the IF filters. Like many tuners the KT-880D uses two ceramic filters in wide IF mode. But while the rated bandwidth of each is 250 kHz, the filters are of different type. One is an MX and the other an MP3, both low-group-delay types. When you cascade two identical filters, group-delay variation doubles because the poles coincide. With nonidentical filters the poles are no longer coincident and the group delay has a chance to spread out. This reduces overall variation and lowers FM distortion.

The front-end uses 3SK122 dual-gate MOSFETs in the RF amplifier and mixer. These micro-X devices, with a noise figure of just 1.2 dB at 200 MHz, likely account for the tuner's sensitivity. While not bulletproof, the front-end resists overload well. I had to turn my antenna toward a 14.5-kW station 2.4 miles away to find any IMD (the resulting signal was huge: 123 dBf, about 400 mV at the antenna terminals). RF overload performance was similar to that of the Pioneer F-90, which sports a balanced mixer. In on-air tests the KT-880D showed less IMD than one F-90, while another F-90 outperformed it.

The LA1235 IF chip feeds this passive postdetection filter that completely eliminates HD Radio self-noise.

This is the postdetection filter response to 200 kHz. The low-frequency roll-off is due to the spectrum analyzer. The unadjusted nulls were at 113.2 and 187.0 kHz.

The LA3401 stereo decoder uses a ceramic resonator and requires no VCO adjustment. It drives passive ultrasonic filters. The filter nulls were almost exactly at 19 kHz and easily adjustable. There is no output buffer following the filters.

The KT-880D has twenty memories in two groups of ten. The memories save neither the IF bandwidth nor the stereo/mono mode. Recalling a complete receiving state may require pushing up to four buttons. Another awkwardness is that a single button controls both stereo/mono and automatic/manual tuning.

The fluorescent display has an interesting way of showing tuning along with signal strength. When correctly tuned, five white bars in a vertical column indicate signal strength. When tuned more than 50 kHz low, red bars to the left respond. When tuned high, red bars to the right take over. This makes manual tuning unambiguous. Automatic tuning puts on a colorful light show.

AM tunes 5301610 kHz and uses an LA1245. There is little audio filtering beyond the 75-s deemphasis common to both FM and AM. AM in the KT-880D isn't exactly crisp, but it sounds less muffled than in some other tuners. A small loop antenna provided plenty of signals in my suburban location.

One KT-880D had adjustable line voltage, deemphasis, and channel spacing. The other had fixed values for the U.S. market. Both had remote-control jacks on the rear panel and detachable line cords with polarized plugs.

The front panel proclaims DIRECT LINEAR RECEPTION CIRCUIT and PENTACLE CIRCUIT. Except for the filter trick, I found no evidence of special circuitry of any kind.

Modifications

In narrow IF mode the KT-880D cascades a single 150-kHz filter with the two wide filters. According to Kenwood, this improves alternate-channel selectivity from 60 to 90 dB. But adjacent-channel selectivity measured only 17 dB. This isn't much. I replaced the single 150 with a pair of 110-kHz filters and an IF amplifier. Adjacent-channel selectivity increased to 40.5 dB, a very worthwhile improvement. Narrow THD remained unchanged at 0.7%. Audio quality sounds the same to me in wide or narrow.

Like many tuners, the KT-880D uses a 1kΩ trimpot ahead of the IF chip to set the stop sensitivity for automatic tuning. But reducing the input signal this way raises the IF noise figure and can prevent the IF chip from limiting on weak signals. I set the pot all the way up. Automatic tuning still worked fine. However, the signal-strength indicator seemed too sensitive. I added 33kΩ in series with R221 in one tuner. The meter segments then illuminated at 17, 37, 48, 66, and 77 dBf, a wide and useful range. The unmodified tuner lit up at 12, 25, 35, 41, and 52 dBf with the pot up full.

The KT-880D has a curious automatic blend function. When a stereo signal drops to about 22 dBf, the tuner strongly blends the left and right channels, leaving only a few dB of separation. At 18 dBf the stereo decoder switches to mono. It's odd that the blend is so aggressive, happens well below the level where stereo noise becomes objectionable, and functions over an RF signal level range of only 4 dB. A Yamaha T-1020 did something similar. Perhaps the intent is to illuminate the stereo indicator as soon as possible, just a few dB above the mono 50-dB quieting point. But I'd rather have the blend take place closer to 50-dB stereo quieting, where it would really do some good, and I'd like it to retain more separation. It is easy enough to alter the threshold and separation by changing R73 and R104. Shorting R76 disables the blend entirely. This 24kΩ resistor is just to the rear of the center slug in the postdetection filter.

The rear panel has a European-style Belling-Lee antenna jack. It accepts a push-on F-connector, but a push-on adapter requires additional rear clearance. It took about an hour of disassembly, drilling, and filing to replace the jack with an F-connector.

The AC line connects to a PCB mounted upside-down on the power transformer. The PCB traces are exposed and I got bit a couple times while working on the tuner. I finally taped a clear plastic shield to the PCB to cover the traces.

The KT-880D has hidden capability. There are spare inverter, flip-flop, op-amp, fluorescent driver, and display sections. I experimentally illuminated REC CAL and can just make out MAIN, SUB, and even BILINGUAL! You might use one of these to indicate that the automatic blend function is engaged.

Alignment

The alignment instructions in the service manual are incomplete. They don't mention L5, the mixer output transformer. Adjust it for lowest stereo distortion. The instructions also neglect L8, which has an adjustable core, but tweaking it had absolutely no effect on anything. The adjustable postdetection filter L18 and ultrasonic filters L19 aren't mentioned, but my filters were so accurately aligned that I think you can ignore them.

The local oscillator is in a shielded compartment. The inductor and capacitor are adjustable, but the access holes in the shield cover were mispunched in both tuners and didn't come close to lining up with the adjustments. I saw no change in varactor tuning voltage when I pried off the cover to gain access, so I didn't bother to drill new holes.

Front-end alignment is supposed to be done by first adjusting the local oscillator for a tuning voltage of 3.0 V at 87.5 MHz and 23.0 V at 108 MHz. Then you are supposed to peak the slugs in the three RF coils for maximum at 98 MHz. The tuned-circuit capacitances aren't adjustable so I checked the tracking at 90 and 106 MHz using the LA1235 signal-meter output. Allowing the varactor voltage to go to 25 V at 108 MHz increased sensitivity in one unit. The signal limits somewhere near 27 V, and because it may vary with temperature, it's best to stay away from the limit.

Do not hang a scope probe on R74 to pick up the LA1235 signal-meter output. The adjacent jumper is at 8 V. I blew the IC drive circuit in one tuner with sloppy probe handling. Pick up the signal at D43, where the adjacent components carry the same signal.

Front-end return loss measured about 15 dB, equivalent to an input SWR of 1.43 and a mismatch loss of just 0.14 dB. Tuning 100 kHz off frequency over most of the FM band degraded the return loss, implying that the varactor drive to the RF input circuit was just about right.

A schematic is here.

Measurements

For the following FM measurements I used IEEE 185-1975, updated as described here. I used the test equipment listed here. The tuner had two 110-kHz filters installed in narrow.

50-dB quieting sensitivity, mono     W 16.5 dBf, N 17.9 dBf
50-dB quieting sensitivity, stereo   W 38.8 dBf, N 37.4 dBf
Adjacent-channel selectivity         N 40.5 dB
THD, 1 kHz, stereo                   W 0.02%, N 0.7%
Stereo separation, 1 kHz             W 53 dB, N 21 dB
S/N, 65 dBf, mono                    W 82 dB
S/N, 65 dBf, stereo                  W 75 dB
S/N, 85 dBf, stereo                  W 79 dB
AM suppression ratio                 W 75 dB, N 75 dB
Capture ratio, stereo, 30 dB         W 13.5 dB, N 12.5 dB
Capture ratio, stereo, 50 dB         W 33.5 dB, N 32.5 dB
RF intermod                          W 86 dBf     (97.7 + 98.5 ⇒ 96.9)
RF spur                              W 106 dBf    (97.74 ⇒ 96.9)
RF image                             W 111.5 dBf  (118.3 ⇒ 96.9)
Noise figure, 96.9 MHz               4.6 dB
Modulation acceptance                W 230%, N 190%
Minimum stereo pilot injection       W 2.8%, N 3.0%
Treble response                      +0.2/-0.9 dB
Bass response, -1 dB                 < 10 Hz
Output impedance, 15 kHz             2.8kΩ
Output level                         W 0.7 V, N 0.7 V
Output level drop, stereo W -> N     0.8 dB

The sensitivity of my tuner varied with temperature. The sensitivity figures above are with the cover off at 25 C ambient. Mono wide sensitivity went to 17.0 dBf with the cover in place. It was 15.0 dBf on a cold morning when I made the noise figure measurement.

The output level spec is for mono. The output level drop in stereo is typical for narrow IF filters. I've seen up to 3 dB for tuners that lower the L+R injection level in narrow to improve stereo separation.


January 15, 201388108 MHz